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Reaction of Allyl Radical with Acetylene

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Reaction of Allyl Radical with Acetylene 石 油 学 会 誌 J. Japan Petrol. Inst., 23, (2), 133-138 (1980) 133 Reaction of Allyl Radical with Acetylene Daisuke NOHARA* and Tomoya SAKAI* The title reaction was performed at 580-740℃, employing biallyl as the source of the allyl radical. The main product of addition of allyl radical to acetylene was cyclopentadiene, and the selectivity attained was more than 90 percent of the total C5 products. Such acyclic product as 1,4-pentadiene was scarcely formed in distinct contrast with the result of the addition of allyl radical to ethylene, in which almost equal amounts of 1-pentene and cyclopentene were formed. The product distribution of the title reaction was reported, and a scheme for the product forma- tion was proposed. From the kinetic analysis, the overall activation energy of reaction for the formation of cyclopentadiene from allyl radical and acetylene was estimated. The present experiment on thermal reaction of 1. Introduction biallyl with acetylene was undertaken to examine Allyl radical has been suggested as playing the the possibility of cycloaddition of allyl radical to role of diene1) in the cyclization reaction with the triple bond. Cyclopentadiene, one of the ex- olefins to form C5-cyclic products analogous to pected C5 compounds in the present reaction, was butadiene which thermally combines with olefins formed in high selectivity, i.e., ca. 40 percent of to yield C6-cyclic compounds. In the pyrolysis all products, including the products from the thermal of ethylene2), C6-cyclic compounds were formed reaction of acetylene itself. The selectivity was mostly through the Diels-Alder reaction between formed to be as high as 60 percent, excluding the butadiene and ethylene, the butadiene being one products from the reaction of acetylene itself. A of the primary products of the reaction. characteristic feature of the reaction of allyl radical with the triple bond was that an acyclic C5 com- ≪+||→〇→-2H2○ pound such as 1,4-pentadiene was scarcely formed. This result was quite different from the result On the other hand, greater amounts of C5-cyclic obtained in the thermal reaction of biallyl in excess compounds were produced in the pyrolysis of pro- ethylene5), where the amount of 1-pentene was as pylene3) than in the case of ethylene. It was abundant as that of cyclopentene. suggested that, in the case of propylene, the dif- ference originated from the contribution of the 2. Experimental allyl radical that was formed through the following The apparatus used was an ordinary atmospheric reaction: flow system. The reactor was made of a quartz annular cylinder. Reactants and products were ◇+◇→◇,◇+H・ analyzed mostly with a FID gas chromatograph equipped with a di-n-butylmaleate capillary column. In our study on the pyrolysis of 1,5-hexadiene Identification of products was confirmed by GC- (biallyl)4), C5-cyclic compounds such as cyclopen- MS. Reaction temperatures were in the range tadiene and cyclopentene were produced as the from 580℃ to 740℃, and residence times were primary products. It is considered that these com- 0.09 to 0.3sec. Equivalent reactor volumes at the pounds were formed by the addition of allyl radical temperatures employed were calculated by the meth- to biallyl. Furthermore, in the pyrolysis of biallyl od of Hougen and Watson6). Acetylene, freed in excess ethylene5), cyclopentene and 1-pentene from acetone vapor, was diluted with deoxygenated were formed with fairly good selectivities, i.e., the nitrogen to around 4vol%. Biallyl vapor was total amount of the two products was ca. 60% mixed with the acetylene-N2 flow and fed into of all products produced. the reactor. Biallyl concentrations were adjusted Received June 18, 1979. * Dept, of Chemical Reaction Engineering, Faculty of around 0.16vol% of the inlet gas. The thermal Pharmaceutical Sciences, Nagoya City University (3-1, reaction of acetylene without biallyl vapor was Tanabedori, Mizuho-ku, Nagoya 467) conducted as a blank test. 石 油 学 会 誌 J. Japan Petrol. Inst., Vol. 23, No. 2, 1980 134 (a) (b) Fig. 1 Product Formation Curve in the Thermal Reaction of Acetylene in terms of apparent activation energy was 110 3. Results and Discussion or 54kJmol-1 without the addition of biallyl; As blank reference for the title reaction, the with the addition of biallyl, however, it changed results and discussion on the thermal reaction of to 150 or 130kJmol-1, respectively. These facts acetylene-N2 will first be briefly described. It will reflect the complexity in the latter case, i.e., the then be followed by a description of the results intermediate radicals in one chain reaction system and discussion of the title reaction of allyl radical affect the reactions of the other system. Common with acetylene. intermediate radicals such as ethynyl and vinyl 3.1 Thermal Reaction of Acetylene are expected to be involved in the chains of both The products obtained in the thermal reaction systems. of acetylene under the present conditions used, 3.2 Reaction of Allyl Radical with Acetylene i.e., ca. 4vol% acetylene in N2 at 580-740℃ Fig. 2 shows the Arrhenius plots for the biallyl for 0.09-0.3sec, were vinylacetylene, benzene and decomposition in the thermal reaction of acetylene- 1,3,5-hexatriene. The presence of the last com- biallyl-N2 gas mixtures that fitted to a first-order pound was indicated from the retention time of rate equation similar to the ethylene case reported glc, but the amount was too small to be identified previously5). The runs conducted at 740℃ (1/T= by GC-MS. 0.99×10-3K-1) were excluded from the figure since The temperatures used in the present experiments were in the lower temperature range classified by Back7) in his study on acetylene pyrolysis where the formation of vinylacetylene and benzene was claimed as the main products7),8). In the lower part of the present temperature range used, poly- merization seemed to become predominant7). Rela- tions between product concentrations and residence times are illustrated in Fig. 1 (a). The presence of an induction period was evident as in other reports9),10). The time courses of concentrations of vinylacetylene and benzene with the addition of biallyl vapor are shown in Fig. 1 (b). The induc- tion period was shortened as observed by many investigators who had pointed out the same trend11),12) when another appropriate radical source existed. The temperature dependency of the rate of formation of vinylacetylene or benzene expressed Fig. 2 Arrhenius Plot for the Biallyl Decomposition 石 油 学 会 誌 J. Japan Petrol. Inst., Vol. 23, No. 2, 1980 135 Table 1 Typical Experimental Data ○: Cyclopentadiene, ○: Propylene, ●: Ethylene, ○: Butadiene, ○: Butene, ○: 1-Pentene-4-yne, ○: 1,4-Pentadiene, ○: Benzene, ○: Vinylacetylene, ●: 1,3,5-Hexatriene Fig. 4 Effect of Biallyl Concentration on Molar Amounts of Products ○: Cyclopentadiene, ○: Propylene, ●: Ethylene, ○: Butadiene, ○: Butene, ○: 1-Pentene-4-yne, ○: 1,4-Pentadiene, ○: Benzene, ○: Vinylacetylene, listed in Table 1. ●: 1,3,5-Hexatriene The product distribution of this reaction is shown Fig. 3 Product Distribution in the Reaction of Allyl in Fig. 3. It is clear that the amount of cyclopen- Radical and Acetylene tadiene was ca. 40% of all products, while that of biallyl was completely decomposed at that tempera- 1,4-pentadiene was less than 3%. In the products, ture. From this figure, kd=1012.5exp (-209,000/ excluding vinylacetylene and benzene, about 60% RT)s-1 was obtained. The kinetic parameters were of all products was cyclopentadiene. The present in numerical agreement with those observed in the result contrasted sharply with that observed in the ethylene-biallyl case5), indicating that decomposi- ethylene-biallyl system in which 1-pentene was form- tion of biallyl was unaffected by the presence of ed in the amount nearly equal to that of cyclo- either ethylene or acetylene. pentene5). The striking difference indicates that The main products in the reaction of allyl radical the cyclization of C=C-C-C=C. radical followed with acetylene were cyclopentadiene, propylene, by hydrogen atom elimination to produce ◇ is ethylene, butadiene and 1-butene, in addition to vinylacetylene and benzene; and the minor prod- much faster than hydrogen abstraction by the same ucts were 1,4-pentadiene, 1-pentene-4-yne, 1,3,5- radical to produce acyclic product, presumably, hexatriene and methane. Very small amounts of because of the absence of available H-donors. unidentified products were also detected in the Illustrated in Fig. 4 is the effect of biallyl con- C5 to C6 fractions. Typical experimental data are centration on the product distribution observed at 石 油 学 会 誌 J. Japan Petrol. Inst., Vol. 23, No. 2, 1980 136 660℃ and 0.17s. The relative amounts of prod- of this radical eliminates hydrogen to produce ucts, excluding vinylacetylene, benzene and 1,3,5- cyclopentadiene according to Reaction (9). hexatriene, which were formed from the thermal ・◇→◇+H・ (9) reaction of acetylene, increased distinctly with bi- allyl concentration, indicating clearly that cyclo- pentadiene was formed through the addition of The hydrogen abstraction product, cyclopentene, allyl radical to acetylene. was not detected in the products. According to 3.2.1 Scheme for Product Formations the above, only small amounts of hydrogen-donating The following scheme is proposed for the reac- compounds, other than propylene, are formed. tions of acetylene-biallyl-N2 gas mixtures. Biallyl This behavior is the characteristic feature of a initially decomposes to produce two allyl radicals system involving acetylene. The H. released in in Reaction (1). Reaction (9) adds to acetylene or other compounds, C=C-C-C-C=C→2C=C-C・ (1) thereby enhancing polymerization and the forma- The fate of the allyl radical thus produced will tion of C2-C4 unsaturated compounds.
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